1. Field of the Invention
The present invention relates to document transport devices; more particularly, to drum type document transport devices wherein a sheet of flexible material (such as paper) is attached to the drum for transport and for processing.
2. Prior Art
The use of a rotary drum for transporting sheet-like material is well known in the prior art. The rotary drums are often used in printing systems. In addition to the transport function, these drums support the sheet-like material during the printing process. Prior art printing systems are further fitted with paper handling mechanisms which load and unload a sheet of paper onto the drum.
A necessary component of the prior art print system is the means used to attach the sheet onto the drum. The prior art often used mechanical fingers for clamping sheets onto the drum. By way of example, U.S. Pat. No. 2,451,079 describes a rotary drum for supporting a sheet in a facsimile printing system. The drum is fitted with two linear rows of pins. The rows of pins are spaced circumferentially and extend outwardly from the surface of the drum. One row of pins releasably secures the leading edge of the sheet while the other row releasably secures the trailing edge of the sheet. A loading plate and a stripper bar are positioned relative to the drum. The loading plate loads a sheet onto the drum while the stripper bar strips a sheet from the drum.
Although the above-described mechanical clamping system works satisfactorily for its intended purpose, the system tends to be relatively slow and complex. The slowness stems from the fact that the response time in which the mechanical system clamps and releases a sheet is relatively long. As such, the print drums using mechanical fingers for gripping the sheet are used with relatively low performance printing systems.
For high performance printing systems, the prior art generally uses pneumatics and/or electrostatic means for tacking the sheet onto the drum. Prior art printing systems generally use coronas as the source for generating the electrostatic force. An example of a prior art printing system using a combination of pneumatics and corona for attaching a sheet onto a print drum is disclosed in the above-referenced Korte patent application.
As for pneumatic systems, the general scheme is to use a segmented drum to transport the sheet. Vacuum for attaching and/or dislodging the sheet is selectively applied to various zones or segments on the drum. The drum is referred to as being segmented because at times during the operation of the system, segments of the drum may or may not have vacuum present.
U.S. Pat. No. 3,545,746 is an example of the prior art segmented drum. The patent describes a document transport consisting of a hollow cylindrical transport drum and document loading and unloading means disposed relative thereto. The cylindrical surface of the drum is fitted with longitudinal and circumferential slots. The inside of the drum is vented to atmosphere by communicating holes. A static partition divides the interior of the drum into two pneumatically independent compartments. By rotating the drum and applying a vacuum to one of the compartments, a document can be carried around with it, to a limited extent, determined by the size of the evacuated compartment.
U.S. Pat. No. 4,145,040 is another example of the prior art segmented type vacuum drum. The drum is adapted for transporting flexible sheets. The drum is fabricated with an active suction zone or sector for gripping the sheets. The drum consists of an inner stationary cylindrical member and an outer rotary cylindrical member. The stationary member is fitted with a suction source and a pressure source. Both sources are displaced relative to each other about the circumference of the stationary cylindrical member. The suction source is vented through a groove to the outside surface of the inner stationary member. Likewise, the pressure source is vented through a recess formed on the surface of the inner stationary member. A sector of the outer rotary member is fitted with rows of apertures. The apertures interconnect the inside surface of the stationary member to the outside surface. A common duct interconnects a row of apertures to the groove or the recess. Each duct is fitted with a piston. The piston controls the pressure (negative or positive) to the apertures. Vacuum (negative pressure) and/or puffs of air (positive pressure) is applied to the sector of the drum as the outer member is rotated relative to the inner.
It is also well known in the prior art to use valves as a means to control vacuum flow to the active segment of the drum. By way of example, U.S. Pat. Nos. 3,663,012 and 3,466,029 describes sectored vacuum transport drums wherein valves are used to control vacuum to the active sector of the drum.
Although the use of pneumatics or a combination of pneumatics and electrostatics is a significant improvement over the use of mechanical gadgets for tacking sheets onto a drum, the prior art pneumatic document transport systems still have several disadvantages. In the first instance, the segmented drum design tends to be complex. The complexity increases as the number of sheet sizes, which the transport system handles, increases.
A complex valving system is generally needed in the prior art pneumatic systems. The valving system is needed to select which port receives flow at any particular time. The valving system increases as the sheet size, which the system handles, increases. Another requirement for a valving system is that the system must know the paper size to enable the supply of vacuum to the proper ports. This requires the intervention of an operator to make a sheet size selection or the use of logic to detect the sheet size. Moreover, with a valving system, one has to use extra care in selecting the valve and in positioning it relative to the drum. Both valve selection and valve positioning are important since the response time of the drum is directly dependent on both variables. Due to their complexity, the prior art pneumatic document transport system has relatively low reliability.
Another prior art problem area is in the type of vacuum system used. As was described previously, the vacuum system is needed to evacuate the drum. Stated another way, the vacuum system creates the force for tacking a sheet onto the drum. The problem in this area stems from the fact that in that type of vacuum system, there is a wide swing in the vacuum between load and no-load conditions on the drum. By way of example, at no-load condition (that is with no paper on the drum), the vacuum is relatively low. At load condition (that is with paper on the drum), the vacuum is substantially higher.
The wide swing in vacuum has several undesirable repercussions. In the first instance, there is a large mismatch between the vacuum requirements to attach and retain a sheet onto the drum. Generally, a relatively high flow is required for attachment, but a relatively low vacuum force is required for retainment. As such, there is a need to recognize that there is a close relationship between the vacuum system which generates the vacuum requirements and to design the vacuum system to minimize the mismatch.
So far, the prior art has failed to recognize and address the interrelation between the vacuum requirements (at the drum) and the design of the vacuum system.
In addition, the high vacuum tends to damage a sheet on the drum. More important, in some types of application, such as ink jet printing, the high vacuum is totally unacceptable. The reason is that the high vacuum sucks the ink through the paper. The prior art attempts to solve the problem by using a relief valve to reduce the pressure at the drum. Needless to say, the use of the relief valve tends to complicate the system and increase cost.